The invention relates to a dielectric fluid, in particular a dielectric fluid for a capacitor.
Capacitors are electrical devices that can be used to store an electrical charge. A capacitor can include a capacitor pack containing conducting plates separated by a non-conductive material, such as a polymer film. The conducing plates and polymer film can be rolled to form windings, which can be housed within a casing, such as a metal or plastic housing. The casing protects and electrically isolates the windings from the environment. In power factor correction capacitors, the windings are typically immersed in a dielectric fluid. The dielectric fluid serves as an insulating material that can help prevent charge breakdown between plates in the capacitor. If these spaces are not filled with a suitable dielectric material, partial discharge can occur under electrical stress, leading to device failure. Conventional dielectric fluids include phenyl orthoxylyl ethane, isopropyl biphenyl, a mixture of methyl diphenyl ethane and lower alkyl diphenyl, mono and dibenzyltoluene, or phenyl xylyl ethane.
A dielectric fluid that provides improved performance in capacitors contains 65% by weight or more monobenzyltoluene. Capacitors including the dielectric fluid can have higher discharge extinction voltages at 60xc2x0 C., and can have increased failure voltages in comparison to capacitors made using a fluid including 60% by weight monobenzyltoluene and 40% by weight diphenylethane. The dielectric fluid can have a low viscosity and low vapor pressure.
In one general aspect, an electrical capacitor includes a casing and a dielectric fluid in the casing. The dielectric fluid includes 65 to 95% by weight of monobenzyltoluene and 5 to 35% by weight of diphenylethane.
In another general aspect, an electrical capacitor includes a dielectric fluid consisting essentially of 65 to 95% by weight of monobenzyltoluene, 5 to 35% by weight of diphenylethane, and up to 5% by weight of a scavenger.
In another general aspect, a dielectric fluid includes 65 to 95% by weight of monobenzyltoluene, 5 to 35% by weight of diphenylethane, and up to 5% by weight of a scavenger.
In another general aspect, a method of making an electrical capacitor including adding a dielectric fluid to a casing. The dielectric fluid includes 60 to 95% by weight of monobenzyltoluene and 5 to 40% by weight of diphenylethane. The dielectric fluid can be added with the casing under reduced pressure. Prior to addition, the dielectric fluid can be heated under reduced pressure.
In another general aspect, an electrical capacitor includes a casing and a dielectric fluid in the casing. The dielectric fluid includes monobenzyltoluene and diphenylethane. The capacitor has a rated voltage and fails a 55xc2x0 C. step stress test at greater than 180% of the rated voltage. The capacitor can fail a xe2x88x9240xc2x0 C. step stress test at greater than 160% of the rated voltage.
The dielectric fluid can include 70 to 90% by weight of monobenzyltoluene and 10 to 30% by weight of diphenylethane. The dielectric fluid also can include up to 5% by weight of a scavenger, or, in some implementations, 0.01 to 2% by weight of a scavenger.
The capacitor can include capacitor packs disposed within the casing. The capacitor can have an extinction voltage of greater than 3.0 kV at 60xc2x0 C.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.